Ordinary Internet users, American and non-American alike, far outnumber legally targeted foreigners in the communications intercepted by the National Security Agency from US digital networks, according to a four-month investigation by The Washington Post. Nine of 10 account holders found in a large cache of intercepted conversations, which former NSA contractor Edward Snowden provided in full to The Post, were not the intended surveillance targets but were caught in a net the agency had cast for somebody else. Many of them were Americans. Nearly half of the surveillance files, a strikingly high proportion, contained names, e-mail addresses or other details that the NSA marked as belonging to US citizens or residents. NSA analysts masked, or “minimized,” more than 65,000 such references to protect Americans’ privacy, but The Post found nearly 900 additional e-mail addresses, unmasked in the files, that could be strongly linked to US citizens or US residents. The surveillance files highlight a policy dilemma that has been aired only abstractly in public. There are discoveries of considerable intelligence value in the intercepted messages -- and collateral harm to privacy on a scale that the Obama Administration has not been willing to address. Among the most valuable contents -- which The Post will not describe in detail, to avoid interfering with ongoing operations -- are fresh revelations about a secret overseas nuclear project, double-dealing by an ostensible ally, a military calamity that befell an unfriendly power, and the identities of aggressive intruders into US computer networks.